RaceCarMove.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class RaceCarMove : MonoBehaviour {

    Rigidbody rb;
    public float fAccel = 15f;
    public float lrAccel = 3f;
    public float turnSpeed = .5f;

    public float breakStrength = .1f;

    //keys
    public KeyCode kForward = KeyCode.W;
    public KeyCode kLeft = KeyCode.A;
    public KeyCode kRight = KeyCode.D;
    public KeyCode kBreak = KeyCode.C;



    void Start ()
    {
        rb = GetComponent<Rigidbody>();
	}

    RaycastHit rayHit;

    bool GroundNormal(out Vector3 normal)
    {
        
        Ray r = new Ray(transform.position, transform.up * -1f);
        if(Physics.Raycast(r.origin, r.direction, out rayHit, 5f))
        {
            normal = rayHit.normal;
            return true;
        }

        normal = Vector3.zero;
        return false;
    }

    //credit: https://math.stackexchange.com/questions/633181/formula-to-project-a-vector-onto-a-plane
    Vector3 ProjectedForwardDirFromGroundNormal()
    {
        var fw = transform.forward;
        Vector3 normal;
        if(GroundNormal(out normal))
        {
            // projection of forward vec onto the plane normal
            var nProjection = Vector3.Dot(fw, normal) * normal;

            // subtract the projection from the foward vec
            return fw - nProjection;
        }

        return fw;
    }

    //private void OnDrawGizmos()
    //{
    //    Gizmos.color = Color.red;
    //    var from = transform.position + Vector3.up * 1.5f;
    //    Vector3 normal;
    //    GroundNormal(out normal);
    //    Gizmos.DrawLine(from, from + normal * 3f);

    //    Gizmos.color = Color.green;
    //    Gizmos.DrawLine(from, from + ProjectedForwardDirFromGroundNormal() * 3f);
    //}

    void FixedUpdate ()
    {

        if (Input.GetKey(kForward))
        {
            var fw = ProjectedForwardDirFromGroundNormal();
            var push = fw * fAccel;
            rb.AddForce(push);
        }

        if (Input.GetKey(KeyCode.D))
        {
            var push = transform.right;
            push.y = 0f;
            push = push.normalized;
            push *= lrAccel;
            rb.AddForce(push, ForceMode.Impulse);
        }

        if(Input.GetKey(KeyCode.A))
        {

            var push = transform.right;
            push.y = 0f;
            push = push.normalized;
            push *= -1f * lrAccel;
            rb.AddForce(push, ForceMode.Impulse);
        }

        //breaks
        if (Input.GetKey(KeyCode.C))
        {
            var push = rb.velocity * breakStrength * -1f;
            push.y = 0f;
            rb.AddForce(push, ForceMode.Impulse);
        }

        if (Mathf.Abs(rb.velocity.x) + Mathf.Abs(rb.velocity.z) > 1f)
        {
            // car / player should 'slerp' towards its own velocity

            Quaternion curRo = transform.rotation;

            Quaternion targetRo = Quaternion.LookRotation(rb.velocity, Vector3.up);

            rb.MoveRotation(Quaternion.Slerp(curRo, targetRo, turnSpeed * Time.deltaTime));
        }
    }
}